I highly recommend the BK14 or 24 if you'd rather D2 over 1095. It'll be right about the perfect size. Perfect size blade for just about any use. Great size handle for smaller hands, yet big enough to where he could still use it as he grows up. I would suggest adding handle scales to it. My preference are TKC g10 scales, but they're about $40 or so. Absolutely worth it, but maybe not at first. There's g10 scales somewhere around 10-$15, I don't recall where I've seen them. And also the black and orange grivory scales that I believe are around the same price, maybe a little less.
Anything around that size will work though.
This is coming from a 5'3"(on a good day) adult.
The factory Grivory/Zytel scales cost ~$15, and you get both the orange and the black for that price.